They'd be just another handset maker, and the competition for Android is brutal. I wouldn't intrinsically consider a MS handset as any more worthwhile than any other handset. If they want to go Android, just produce Android apps. But ultimately it's the stores/subs that make the money (unless you can sell premium hardware to boot) and tying everyone into the Windows ecosystem is the very best way for MS to secure that. Surface 3 has finally seen MS make money from Surface, and it's because S3 is a professional, Windows product running full-fat Windows.
Honestly, anything other than 100% Windows Everywhere is the Wrong Move.
I think if Intel can ever fully execute on their low-power processors, the endgame of a phone running Windows 10 that you can just drop into a dock to get a full desktop PC experience is a very compelling concept.